1. Technical Field
The present invention relates to a wavelength-tunable interference filter that selects light with a desired wavelength from incident light to output it, an optical module having the wavelength-tunable interference filter, and an optical analysis apparatus having the optical module.
2. Related Art
In the related art, there is a wavelength-tunable interference filter that emits light with a desired wavelength by generating multi-interference of light between a pair of reflection films (for example, refer to JP-A-2009-251105).
The optical filter apparatus (wavelength-tunable interference filter) disclosed in JP-A-2009-251105 has a first substrate and a second substrate, each of which is provided with a movable mirror and a fixed mirror on the facing surface.
In the first substrate, the movable mirror is provided in a first portion at the center of the substrate, and a second portion having flexibility with a thickness dimension smaller than that of the first portion is provided on the outer periphery of the first portion. In addition, a first electrode is provided on the surface of the second portion of the first substrate facing the second substrate, and a second electrode facing the first electrode with a predetermined distance is provided on the surface facing the first electrode of the second substrate.
In such a wavelength-tunable interference filter, if a voltage is applied between the first and second electrodes, the second portion of the first substrate is bent toward the second substrate due to an electrostatic attractive force so that a gap dimension varies between the movable mirror and the fixed mirror. As a result, the wavelength-tunable interference filter can extract light with a wavelength corresponding to the gap dimension between the mirrors from the incident light by controlling a voltage between the first and second electrodes.
In the wavelength-tunable filter as disclosed in JP-A-2009-251105, the first electrode is provided in the second portion of the first substrate having flexibility. Although a film-shaped electrode is used as the first electrode, an internal stress is applied in an in-plane direction of the film (a direction along the surface of the first substrate) when such a first electrode is formed. The direction or the size of the internal stress is determined based on a film formation method or a film material. In addition, when the internal stress is applied to a direction toward the center of the film, a compressive stress is applied. When the internal stress is applied to the outer side from the center of the film of the first electrode, a tensile stress is applied. Here, when a compressive stress is applied to the first electrode formed in the first substrate, the first substrate is bent toward the second substrate. When a tensile stress is applied to the first electrode formed in the first substrate, the first substrate is bent to a direction far from the second substrate.
As such, if the first substrate is bent due to the internal stress of the first electrode, the movable reflective film is also bent in response to the bending of the substrate, the movable reflective film and the fixed reflective film may not be maintained in parallel, compared to the initial state in which a drive voltage is not applied between the first and second electrodes, so that the resolution of the wavelength-tunable interference filter may be degraded.